ACTUAL guide number of 380EX

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I have two 380EX speedlites. After much thinking, I have not been able to come up with an accurate guess on what the actual GN of the 380EX is. I have heard elsewhere that all flashes are given a higher GN than it actually has. So does anyone know what the actual ranges of 25, 100 , 400 and 800 ASA films would be?

-- Colin Miller (ckmiller@pond.net), April 24, 1999

Answers

Colin:

Guide Number = f stop x flash-to-subject distance

The "official" GN for the 380 is 100 (in feet, 100 ASA is assumed)

To check to see what the actual GN is, use 100 speed slide film, and take some pictures at 10 feet with the flash on full power manual (is this an option with the 380? I'm not familiar with that flash). According to the official GN, you would need an f stop of 10 (10 ft x f10 = 100 GN). Bracket the pictures around f 10. (Don't use any type of flash modifier, Sto-Fen, Lumiquest, etc; just bare flash). TAKE NOTES.

When you get the slides back, see which setting gave the best exposure. Then plug that fstop into the formula and you'll find the actual GN for your flash. Of course, ceiling height, nearness of walls, color of ceiling and walls will all affect the GN.

TTL flash is a lot easier than messing with GNs and f stops -- I've recently moved from a Vivitar 283 on Canon FD's to an A2e with a 540EZ, and WOW is flash effortless now. But it's still important to understand the theory so you know when you have to out-think the camera.

-- Matt Orth (morth@erols.com), April 24, 1999.


Focal length-GN (in meters, for feet multiply by 3 approximately): 24-21;28-23;35-28;50-31;70-33;105-38.

-- (arief.novisto@vandrbilt.edu), April 24, 1999.

Good stuff above, to which I can add:

There's a guide number chart on page 44 of your 380EX's manual.

In meters, the G.N. is 31 or 102.30 feet. I multiply be 3.3 to convert meters to feet. Keep in mind that your Guide Number changes not only with ASA, but also with focal length. It decreases at the wide end and increases at telephoto. Finally, when using high speed sync (FP Flash) guide numbers plummet (see the chart).

By the way, I get excellent results with both normal and FP flash.The lower guide numbers associated with FP flash are of no consequence in my photography since I use high speed synch only to add a bit of fill to knock out shadows caused by bright, harsh sunlight, as well as to add catchlights to my subjects'eyes.

Great feature: I can shoot wide open to blur out the background to make my portrait subjects stand out while shooting flash synched to any shutter speed up to and including 1/4000th of a second. This is otherwise unheard of in 35mm photography, except with EX flashes used on specific EOS models. Good going, Canon!

-- Roy Kekewich (roykekewich@yahoo.com), January 15, 2000.


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